C3H/STWi mice were highly susceptible (45% in 8.9 mos.) to mammary carcinogenesis by DMBA. All preneoplastic lesions were ductal hyperplasias, rather than hyperplastic alveolar nodules as in MMTV-carcinogenesis and the tumors were largely ductal papillomas. Virus-specific RNA levels were not increased over background in the tumors. Immunologic studies have indicated the noncoordinate expression of virus-specific antigens in these tumors. No cocarcinogenic effect was found in MMTV-infected C3H/StWi mice treated with DMBA. Hybrids between C3H/AvyfB (90% mammary cancer) and C3H/StWi (1% mammary cancer) and backcross progeny, produced by mating C3H/StWi males with F1 females, show significantly reduced mammary cancer incidence, 28 and 8% respectively. Hybridization analysis for MMTV RNA in lactating mammary tissue from each parent strain, the F1 offspring, and the BC1 segregants demonstrated MMTV RNA levels to be 2-3 fold greater in the C3H/StWi glands than in C3H/AvyfB, however no MMTV-specific translation products were detected in C3H/StWi glands. Transcription levels in F1 and BC1 females were indistinguishable, however the presence of immunologically-detectable MMTV structural proteins seemed to segregate with Avy gene.